1. Field of the Invention
The present invention relates to a method of manufacturing a polarizable electrode for an electric double-layer capacitor.
2. Description of the Background Art
A known electric double-layer capacitor will be schematically described with reference to FIGS. 4 and 5.
As shown in FIG. 5, a known electric double-layer capacitor 1 is generally produced from a plurality of, e.g., six capacitor cells 2, which are coaxially stacked and contained in a cup-shaped case 3.
Each capacitor cell 2, provided in the form of a circular plate, as shown in FIG. 4, comprises a pair of polarizable electrodes 4a and 4b, a separator 5, an annular gasket 6, and a pair of current collectors 7a and 7b which are thermally bonded to upper and lower surfaces of the gasket 6 respectively.
More specifically, the polarizable electrodes 4a and 4b, including solid carbonaceous compacts, are isolated from each other by the electrical insulating separator 5. The separator 5, which is made of a polyolefin microporous film or nonwoven fabric, or paper, is preferably provided with a projecting portion along its peripheral edge. This projecting portion encloses the second polarizable electrode 4b, so as to prevent the polarizable electrodes 4a and 4b from shorting across the respective peripheral portions. The polarizable electrodes 4a and 4b and the separator 5 are fixed to each other by adhesives 8 which are applied to parts of the interfaces therebetween. These polarizable electrodes 4a and 4b and separator 5 are impregnated with an electrolyte solution such as an aqueous solution containing 50 percent by weight of sulfuric acid, for example.
The gasket 6 is formed of a substrate 9, made of ethylene propylene valcanized rubber, which is integrally provided on both surfaces with low-density polyethylene layers 10a and 10b, for example. The current collectors 7a and 7b are formed of polyethylene films which are made conductive by adding a carbon material such as carbon black, for example, to serve as electrical conduction means for the polarizable electrodes 4a and 4b, as well as to airtightly seal the polarizable electrodes 4a and 4b and the separator 5 within the gasket 6.
A required number of, e.g., six capacitor cells 2 having the aforementioned structure are stacked in order to provide a desired rated voltage, as shown in FIG. 5. An electric insulating thermal contraction tube 11 encloses the stacked capacitor cells 2, thereby integrating the same. A cell assembly 12 obtained by such integration is contained in the case 3 under pressure.
Elastic conductive plates 13a and 13b of highly conductive resin or rubber are arranged in the case 3 to be in contact with the upper and lower surfaces of the cell assembly 12 respectively. A terminal assembly 14 is provided on the upper elastic conductive plate 13a. This terminal assembly 14 comprises two terminals 15a and 15b, formed of metal plates, which are attached to each other by an insulating plate 16. The terminal 15a is electrically connected to the upper surface of the cell assembly 12 through the elastic conductive plate 13a. The terminal 15b, which is in contact with the upper edge of the case 3, is electrically connected to the lower surface of the cell assembly 12 through the case 3 and the elastic conductive plate 13b. As understood from such electrical connection arrangements, the elastic conductive plates 13a and 13b are adapted to reduce contact resistances between the upper surface of the cell assembly 12 and the terminal 15a, and between the lower surface of the cell assembly 12 and the bottom surface of the case 3 respectively, thereby stabilizing the electrical connection.
A sealing resin member 17 is applied to cover the opening portion of the case 3. This sealing resin member 17 airtightly forms a seal for the case 3 to supply the electric double-layer capacitor 1 with washability etc. Another thermal contraction tube 18 covers the outer peripheral surface of the case 3, to insulate the same.
In order to reduce the size of the aforementioned electric double-layer capacitor 1 particularly in the vertical direction, i.e., to reduce the height H shown in FIG. 5, it is necessary to reduce the capacitor cells 2, forming the cell assembly 12, in thickness. The capacitor cells 2 are most effectively reduced in thickness by a method of forming the polarizable electrodes 4a and 4b as sheets.
In general, sheet type polarizable electrodes are manufactured by shaping a rubber-like viscous admixture, which is composed of polytetrafluoroethylene (PTFE) resin and a liquid lubricant, into sheets by rolling. The liquid lubricant is prepared from water, alcohol, glycol or the like.
When the sheets are reduced in thickness in the aforementioned method, however, the sheets overlappingly stick to each other due to elongation caused by rubber elasticity of the admixture and adhesion of the liquid lubricant, to make handling substantially impossible. Thus, the minimum size of a manufacturable sheet has been limited to 0.6 mm in thickness.
In order to solve the aforementioned problem, it may be considered to first obtain a sheet-type preform having a certain degree of thickness, then remove the liquid lubricant from the preform, and thereafter finally shape the preform into a thin film by rolling. In this method, however, it is still impossible to manufacture a film which is smaller than 0.6 mm in thickness, due to cracking, fracturing etc. caused by rolling.
To this end, there has been proposed a method of removing the liquid lubricant from a sheet-type preform and thereafter uniaxially or multiaxially drawing the preform, as disclosed in Japanese Patent Laying-Open No. 107011/1988.
According to this method, it is possible to manufacture a sheet which is smaller than 0.6 mm in thickness. However, this is not an industrially proper method since the steps are complicated by the drawing step which is carried out after the rolling step and a long time is required for the drawing step itself.